1. Field of the Invention
The present invention relates to carbon fiber-reinforced carbon composite materials, processes for their production and first walls of nuclear fusion reactors made of such carbon composite materials.
2. Discussion of the Background
Carbon fiber-reinforced carbon composite materials (hereinafter referred to simply as C/C composite materials) are light in weight and highly strong and have a feature that they are excellent in the heat resistance and corrosion resistance. Therefore, they are used, for example, for aerospace materials such as rocket nozzles, nose cones or disk brakes of air planes, for heater elements, for hot pressing molds, for other mechanical parts, and for parts of nuclear reactors.
Such C/C composite materials are usually prepared by impregnating or mixing a matrix material such as a thermosetting resin such as a phenol resin or a furan resin, or a thermoplastic resin such as pitch, to long or short carbon fibers of e.g. polyacrylonitrile or pitch type, followed by heating and molding, then baking the molded product in a non-oxidizing atmosphere such as an inert gas atmosphere at a temperature of from 600.degree. to 1,000.degree. C., and further densifying-the product by impregnating pitch or a resin thereto, followed by baking, or by a chemical vapor deposition method, or a combination of such methods, followed, if necessary, by graphitization.
However, the resulting C/C composite materials were not necessarily satisfactory when they were used for the purposes of conducting or removing heat in one direction i.e. in the thickness direction, and they had problems in their practical application.
For example, a first wall of a nuclear fusion reactor represents the entire structure in the nuclear fusion reactor, which is disposed to face the plasma, and includes e.g. limiters, diverters, blankets, and parts thereof. Such first wall is disposed close to the plasma and thus is under a severe environmental condition such that it is subjected to heat from plasma and bombernment of plasma particles. Particularly, the limiters and diverters receive high temperature loads, whereby the heat load conditions are particularly severe. As one of materials used for the first wall under such severe conditions, graphite may be mentioned. Graphite is an excellent low atomic number material from the viewpoint of plasma impurities and also has high thermal shock resistance.
FIG. 4 in the drawings illustrates the most typical conventional first wall wherein graphite is used. In the illustrated first wall, a graphite tile 11 is secured to a metal substrate 3 by means of a fixing plate 8 and a connector 9. When heat from the plasma enters the graphite tile 11 facing the plasma, the heat is conducted to the substrate by the contact thermal conduction and also dissipated by thermal radiation. In such system, the graphite tile 11 and the substrate 3 are in contact with each other merely by the mechanical connection, and the thermal conductivity at the contact portion is not adequate, and cooling tends to be inadequate when the heat load is high or lasts for a long period of time. The conventional first wall has the following problems.
When a high heat load (for example, 2 km/cm.sup.2 for 3 seconds, or 4 km/cm.sup.2 for at least one second) is exerted to the first wall, the surface temperature will be as high as at least about 2,800.degree. C. and the vapor pressure of the graphite tile will be at least about 10.sup.-3 atm, whereby the loss in thickness by sublimation from the surface of the graphite tile will be as large as about a few tends .mu.m/sec. As a result, inclusion of carbon atoms in the plasma increases, which brings about a problem that the control of plasma impurities will be thereby seriously adversely affected. Further, the loss of the graphite surface is substantial, whereby there is a problem that the useful life of the first wall is short.
In conventional nuclear fusion reactors, it is rare that such high heat load is exerted to the first wall, and the conventional first wall may sufficiently provide its function against the above-mentioned problems. However, in order to further improve the level of safety, or for a future nuclear fusion reactor for which it is expected that a heat load higher than ever will be exerted to the first wall constantly over a long period of time, it is desired to develop a first wall having the above problems adequately solved.
Under these circumstances, the present inventors have conducted various studies to overcome the above-mentioned drawbacks and to obtain a C/C composite material useful for the above-mentioned first wall or the like, and have finally arrived at the present invention.